There was an attempt at a script that would achieve that, but there's certain things about the structure of the combination file that can make it very difficult to merge two files that weren't created from the exact same source. And the result certainly wouldn't be something that I would recommend submitting to KDP.

You'd be better served using the existing (official) methods to tailor/tweak the individual KF8/MOBI portions of the combined file when it's being built by kindlegen.